The Flint Heart is described as "freely abridged from the 1910 story by Eden Phillpotts" by Katherine and John Paterson. I haven't been quite able to discover exactly how much abridgement and how much rewriting has been involved, and I haven't read the original story, either (though I guess that technically I could -- it is still out there), but regardless, the authors have impressively retained the flavour of children's stories from the opening of the 20th century.
In short, the novel deals with the creation and influence of a flint heart, which hardens the heart of its owner and causes them to become callous and greedy. Fairies, animals, kind-hearted children, and a hot water bottle made in Germany all figure into the adventure.
It's interesting that what I like most about the book may also limit its audience. This reworking, though published in 2011, has retained the feel and voice of children's stories a century ago (and according to reviews, it's retained much of the original text, as well). I'm not sure how the average young reader (or parent of the average young reader) today will handle the vocabulary and rather elegant turns of phrase that are largely responsible for the book's charm.
Adults may find the episodic nature of the book tiresome, but I suspect that kids will be more accepting of it. Word is it's being adapted into a film, so I'm interested in whether that adaptation will retain the tone of the storytelling. I'm not sure how much a more contemporary style would help (sales) or hinder (the heart) of the story.